Mike, I'm afraid you're wrong!
The Cuscuna/Ruppli BN Label discography book (first edition 1988) states that the first session recorded by BN in both mono and stereo was the May 8, 1957 session (Horace Silver 'The Stylings of Silver'). Or has this Bible been proved wrong?
I have this from a note by Cuscuna himself on some reissue he produced! (I think it was the Japanese 3 LP boy "The Other Side of Blue Note 1500 Series".) As I said, the Sabu was only released in mono at the time - maybe the Silver was the first Blue Note to be issued in both mono and stereo.
I had the Japanese LP reissue Cuscuna produced back then and now have the Japanese CD - clearly stereo.
Perhaps the question should be which was the first Blue Note stereo session engineered by Rudy Van Gelder? A note at the end of the Silver date (May 8, 1957) in the Ruppli book clearly states that this is Blue Note's first stereo date. The Sabu date (April 28, 1957) was not recorded at Rudy's, it was recorded at Manhattan Towers, NYC, engineer uncredited. I've always assumed that the two week period April 21 - May 8) during which Blue Note did not record at Rudy's was while Rudy was installing his stereo recording equipment, which may be true, but a quick check of Ruppli's Prestige discography shows several dates (April 24, 26, 27, May 1, 3) not issued (and probably not recorded) in stereo. And Savoy recorded there on May 2, also mono.
Btw, "The Stylings of Silver" is definitely not the first Blue Note to be issued in stereo. The stereo version was first released in 1967 when Liberty took over and "stereo-ized" the whole in print catalog - fake stereo on the earlier sessions, true stereo on the later ones. And, in many instances, they were unsure of what they had. I've found any number of Liberty-pressed 4000 series Blue Notes that claim to be "rechanneled for stereo" but which play true stereo.